Objective To determine whether increasing maternal age increases the risk of operative delivery and to investigate whether such a trend is due to fetal or maternal factors. Design Analysis of prospectively collected data on a maternity unit database. Setting A postgraduate teaching hospital. Population 6410 nulliparous women with singleton cephalic pregnancies delivering at term (37-42 weeks of gestation) between 1 January 92 and 31 December 95. Main outcome measures Mode of delivery, rates of prelabour caesarean section, induction of labour and epidural usage. Results There was a positive, highly significant association between increasing maternal age and obstetric intervention. Prelabour (P < 0.001) and emergency (P < 0.001) caesarean section, instrumental vaginal delivery (spontaneous labour P < 0.001; induced labour P = 0.001), induction of labour (P < 0.001) and epidural usage in spontaneous labour (P = 0.005) all increased with increasing age. In the second stage of labour fetal distress and failure to advance, requiring instrumental delivery, were both more likely with increasing maternal age (in both P < 0.001). Epidural usage in induced labour and the incidence of small for gestational age newborns did not increase with increasing maternal age (P = 0.68 and P = 0.50, respectively). Conclusions This study demonstrates that increasing maternal age is associated with an incremental increase in obstetric intervention. Previous studies have demonstrated a significant effect in women older than 35 years of age, but these data show changes on a continuum from teenage years. This finding may reflect a progressive, age-related deterioration in myometrial function